C515A_9708 Datasheet, PDF (86/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6.2.2.3.3 Compare Mode 1
In compare mode 1, the software adaptively determines the transition of the output signal. lt is
commonly used when output signals are not related to a constant signal perlod (as in a standard
PWM Generation) but must be controlled very precisely with high resolution and without jitter. In
compare mode 1, both transitions of a signal can be controlled. Compare outputs in this mode can
be regarded as high speed outputs which are independent of the CPU activity.
lf compare mode 1 is enabled and the software writes to the appropriate output latch at the port, the
new value will not appear at the output pin until the next compare match occurs. Thus, one can
choose whether the output signal is to make a new transition (1-to-0 or 0-to-1, depending on the
actual pinlevel) or should keep its old value at the time the timer 2 count matches the stored
compare value.
Figure 6-19 and figure 6-20 show functional diagrams of the timer/compare register/port latch
configuration in compare mode 1. In this function, the port latch consists of two separate latches.
The upper latch (which acts as a “shadow latch“) can be written under software control, but its value
will only be transferred to the output latch (and thus to the port pin) in response to a compare match.
Note that the double latch structure is transparent as long as the internal compare signal is active.
While the compare signal is active, a write operation to the port will then change both latches. This
may become important when driving timer 2 with a slow external clock. In this case the compare
signal could be active for many machine cycles in which the CPU could unintentionally change the
contents of the port latch.
A read-modify-write instruction will read the user-controlled „shadow latch“ and write the modified
value back to this “shadow-latch“. A standard read instruction will - as usual - read the pin of the
corresponding compare output.
Semiconductor Group
6-35
1997-08-01